One of the problems of audio bridges is that the operation of their voice operated loss control and switching circuit is very much dependent upon the received signal level at each input port to the bridge. If one of the received signal levels is low (because of a long transmission loop for example) compared to the others, it may well cause voice switching problems for its user during a conference call. The conferee either has to speak louder to increase his own signal level or has to suffer a degradation in terms of conference participation and satisfaction. This problem may be eliminated by bringing each bridge input port level to a normalized value using a level normalizing circuit within the bridge. One known arrangement for achieving this is to utilize individual automatic gain control circuits for each of the input ports to the bridge. While such an arrangement is very effective, it is expensive to implement due to the complete duplication of monitor and control equipment for each channel.